It is known that analog to digital converters can be used to digitize an input signal. It is also known to analyze and compare parameters of repetitive wave forms using processing techniques such as Fourier transform analysis. In some forms of analysis users wish to have a constant number of sampling points across a single cycle of the repetitive waveform, such as a sinusoid, even if the fundamental frequency of the waveform varies slightly. Such an analysis may be performed, for example, by power distribution companies in order to monitor the performance of the electricity generating and supply systems. The electricity is supplied at a nominal frequency, typically of around 50 Hz or 60 Hz. However in reality the supply frequency may vary over a limited but allowable range. This requires the rate at which data samples are produced by an analog to digital converter to be varied in order that the same number of sample points occur in a single cycle.